The interior of the Earth is cooling faster than expected, a study has found — meaning our planet will become inactive like Mercury and Mars sooner than thought.

ETH Zürich-led researchers studied the thermal properties of bridgmanite, the primary mineral that makes up the boundary between Earth's mantle and outer core.

The thermal conductivity of this boundary layer determines how much energy can flow from molten iron-nickel core to the much cooler, viscous mantle above it.

Using a laser on a diamond anvil press to simulate core–mantle boundary conditions, the team found that bridgmanite conducts heat 1.5 times better than thought.

This will likely mean that plate tectonics — which is dependent on heat-driven convection in the mantle — will slow down faster than previously thought.

It remains unclear, however, exactly how long this process will take.

The investigation was undertaken by earth scientist Motohiko Murakami of ETH Zürich and his international team of colleagues.

'Our results could give us a new perspective on the evolution of the Earth's dynamics,' said Professor Murakami explains.

'They suggest that Earth, like the other rocky planets Mercury and Mars, is cooling and becoming inactive much faster than expected.'

Estimating how much heat bridgmanite can transfer from the core to the mantle has long been challenging, because experimentally verifying the thermal conductivity of the mineral at such extreme conditions is extremely difficult.

In their study, the team employed a 'optical absorption' measuring system in which a single crystal of bridgmanite was compressed within a diamond anvil cell, heated with one laser, and then probed with another.

'This measurement system let us show that the thermal conductivity of bridgmanite is about 1.5 times higher than assumed,' Professor Murakami explained.

This, by extension, means that the rate at which heat escapes from the core up into the mantle will also be higher than was previously assumed — leading to increased convection of material within the mantle and a more rapidly-cooling Earth.

This cooling rate, the researchers found, may even increase in the future.

This is because when the core–mantle boundary cools beyond a certain point, the mineral phase that is stable at this interface will change from bridgmanite to post-perovskite, which conducts heat even more efficiently than bridgmanite.

What remains unclear, however, is how long it will take for convention currents within the mantle to grind to a halt.

'We still don't know enough about these kinds of events to pin down their timing,' Professor Murakami said — noting that we first need a better understanding of the ways that mantle convection works both spatially and temporally.

Alongside this, the earth scientist added, we would also need to determine how mantle dynamics are affected by the decay of radioactive elements in the core, which is one of the primary sources of the Earth's internal heat.